The concept of using electrically erasable phase change materials, i.e., ones that can be electrically switched between amorphous and crystalline states, for a semiconductor memory device is known in the art as described in U.S. Pat. No. 3,271,591. Such phase change materials, for example, chalcogenide materials such as germanium, tellurium and selenium or combinations thereof, are capable of being switched between a first structural state, wherein the materials are generally amorphous, and a second structural state, wherein the materials are generally crystalline. The phase change materials may also be electrically switched between different detectable states from completely amorphous to completely crystalline states and states therebetween. Therefore, the materials may be switched in incremental steps. A phase change material also generally exhibits different electrical characteristics depending upon its state. For instance, the material may exhibit lower electrical resistivity in the crystalline state than in the amorphous state. Such a change in resistivity may be detected with known current sensing schemes, which, in turn, allows for the storage of “data” in the form of logic “0” or “1”.
In operation, the phase change material is capable of being transformed from a high resistance state to a low resistance state when a pulse of energy, known as “set pulse”, is applied to the material. The energy pulse causes at least a portion of the material to change from an amorphous state to a crystalline state. Additional set pulses may further crystallize the material, thereby decreasing the resistivity of the material.
In U.S. Pat. No. 4,599,705, entitled “Programmable cell for use in Programmable electronic arrays.,” Holmberg et al., describes a programmable cell for use in programmable electronic arrays, such as PROM devices, logic arrays, gate arrays and die interconnect arrays. The programmable cell incorporates a phase change material having a highly non-conductive state settable and substantially non-resettable into a highly conductive state.